Method of controlling the physical dimensions of molded hydrocolloid bodies



Paienied June 24, 1941 12,24asr22 METHOD OF CONTROLLING THE PHYSICALDIIWENSIONS F MOLDED HYDROCOLLOID BODIES Walter J. van Rossem, LosAngelcs, Calif., as-

signor to Surgident, Ltd., LosAngeles, Califr, a corporation ofCalifornia.

No Drawing. Application. April 2'7, .1940,

. Serial No. 332,118

6 Claims.

This invention relates to the control of a molded body of an aqueous gelof the type characterized by hydrocolloid dental impressioncompositions, whereby the physical dimensions of such molded body may becontrolled, modified, or preserved within close limits, and pertainsparticularly to the control of the physical dimensions of moldedhydrocolloids having an agar-agar base.

One of the particular objects of the invention is to provide a methodwhereby the molded gel body may be caused to expand or contract withincertain narrow limits, for the purpose of controlling the physicaldimensions of such gel body in a model reproducing procedure, so that aresultant model diiiering from the physical dimensions of the originalmodel may be obtained.

A further particular object of the invention is to provide a methodwhereby a hydrocolloid impression of a given model may be preserved inits original physical dimensions over a relatively extended period, sothat a faithful reproduction may be obtained of a given subject.

In the use of hydrocolloid dental impression compositions, considerabledifficulty has heretofore been experienced in the proper control of animpression made in the mouth for example, with respect to its physicaldimensions, if any significant delay is had between the time offormation of the impression and the time that the reproducing plaster ispoured into the impression. It will be appreciated that the averagedentist is not in a position to take an impression of the patients mouthand then promptly thereafter pour the plaster into the impression inorder to secure the best possible plaster reproduction. It has beencommon practice therefore to allow the impression to stand for some timebetween the taking of the impression and the pouring of the plaster, andtwo diiferent techniques have been employed in an attempt to preservethe impression in its original physical dimensions. One of thesetechniques has been that of immersing the impression in a body of water;the other has been to store it at room temperature in an atmosphere of100% relative humidity.

Hydrocolloid dental impression compositions, in gel condition, willexpand when immersed in water for any significant length of time, thedegree'of expansion being roughly in direct proportion to the proportionof gel-forming material in the hydrocolloid composition and averagingabout 1% for agar-agar hydrocolloids containing in the neighborhood ofagar-agar. When a hydrocolloid gel material is stored in anatmosproductions are to be made.

phere of humidity, it undergoes what is generally termed synaeresis, bywhich a significant exudation of fluid from the gel body takes place,with a consequent reduction in the physical dimensions of the gel body.After such contraction one might attempt to restore the gel body to itsoriginal physical dimensions by subsequent immersion in water, but theswelling would not be uniform unless the body were allowed to remain inthe water until it was thoroughly saturated, and this would result in anexpansion of the body to greater physical dimensions than those of itsoriginal condition, with no control being available.

According to the present invention, I employ an electrolyte in solutionto control the degree of hydration of a molded gel body and thusindirectly, but effectively, control the physical dimensions of the gelbody. The hydrocolloid impression composition may comprise an aqueousgel of agar-agar with or without added modifying agents or materialssuch as fibrous materials, waxy materials, resins, etc., which may beadded according to the specific use to which the material is to be putin order to improve its physical strength and consistencycharacteristics. In its preferred aspects, the invention contemplatesthe addition of an electrolyte directly to the hydrocolloid material,such electrolyte being added in a proportion such as to produce acontraction of the material upon solidification from sol to gelcondition which is equivalent to the expansion which is exhibited by theplaster from which re- For example, a good grade of dental plaster orstone will have a setting expansion of approximately 0.15 per cent (as aspecific example, one good commercial plaster is sold under a guaranteethat the expansion shall be within the range of +0.10 and +0.15%), andin order to match this expansion I incorporate approximately two percent of potassium sulphate, for example, which will cause a contractionin the solidified molded gel body which results in a uniform dimensionreduction of 0.15 per cent in the impression after it is removed fromaround the subject.

The molded gel body or impression produced from this composition willtherefore be 0.15 per cent smaller than the subject from which theimpression was made, and if these dimensions of the impression can bepreserved until the plaster is poured into the impression for the makingof a model, the subsequent models will be substantially of the same sizeas the original subject owing to the approximately 0.15 per centexpansion which the plaster undergoes in setting. The actual function ofthe electrolyte in causing this contraction of the impressioncomposition at the time of setting or subsequently thereafter is notclearly understood, but in practice it appears that the compositionfirst sets about the subject and immediately after setting undergoes acontraction which results in the production of the reduced sizeimpression. This is verified by the experience of those who have hadimpressions of their mouths taken, inasmuch as they report that there isfirst the sensation of the solidification of the material from sol togel condition and immediately thereafter there is the sensation that theimpression clamps down about the teeth and gums. In order to preservethe impression in the contracted condition, I may immerse the same in anaqueous solution of the same electrolyte as was present in thecomposition at a concentration substantially equivalent to theconcentration of the electrolyte which was present in the compositionwhen the impression was formed and the impression can be maintained inthis solution for a period of 24 to 48 hours without significant loss orgain in dimensions. If the impression is immersed in plain water insteadof in the electrolyte solution, it will undergo expansion as theelectrolyte diffuses out of the gel body, and in a period of 24; hoursit will have expanded to substantially the full dimensions of which itis capable, somewhat within the range above set forth. If the impressionis immersed in a more concentrated solution of the electrolyte, a slightfurther contraction takes place, although the change resulting from anincrease in concentration of the electrolyte beyond about 0.1 normal isquite nominal.

To provide a specific illustration of the accuracy of reproductionobtainable through the practice of this invention, I made six successivecasts from an impression of a standard die provided with two parallelpins spaced one inch on centers, the casts being made with a stand arddental stone (having a guaranteed expansion of 0.10 to 0.15%) atintervals of approximately one hour. ihe dental impression material usedwas of the following approximate composition:

Per cent Agar-agar 15 Balata-like rubber isomer 1 Glycerine 0.3 K2SO4 2Water 82.7 Preservative, coloring and flavoring qs.

One cast was made immediately after the impression was removed from thedie; the molded impression was allowed to stand in the air (thus subjectto moisture loss by evaporation) for approximately one-half hour, toallow the stone to set, the cast withdrawn, and the molded impressionimmersed in a 2% solution of K2604 for the same length of time as it hadbeen exposed to moisture loss during the plaster-setting period. Sixsuccessive casts were made in this manner, and the resulting casts werefound to vary only $013003" from the original die.

As a further illustration, I prepared a mold of the above compositionfrom the same die, produced a cast therefrom promptly, immersed themolded impression in a 2% K2804 solution for 23 hours and made a newcast at that time. This was repeated at 24 hour intervals until sixsuccessive casts were obtained, and the accuracy assesses of reproducingwas almost identical with those of the first six casts, indicating thatprolonged immersion of the molded impression did not cause a swelling ofthe gel body.

Electrolytes other than potassium sulphate are useful according to thisinvention when employed at a concentration in the neighborhood of 0.1normal, it having been observed that the contraction effect due to theelectrolyte is in substantial harmony with the so-called lyotropicseries of cations. Substantially all cations are found to exert someinfluence upon the contraction of the agar-agar gel up to concentrationsin the neighborhood of 0.01 normal, and the effect up to thatconcentration appears to be that of a direct electrostatic effectexerted by the cations on the electro-negative agar gel, depending onlyon the valency, with the exception oi acid electrolytes which aresomewhat stronger in their effect than are trivalent cations. Atconcentrations in excess of .01 normal, and particularly atconcentrations in the neighborhood of 0.1 normal, the lyotropic effectbecomes more important and the valency of the ions loses its importance;for example, the alkali cations lithium, sodium, and potassium divergeinto the well known lyotropic series at concentrations in theneighborhood of 0.1 normal, with potassium exerting the greatestcontracting effect. At the concentrations which I prefer to employ, asgiven above in my specific example in connection with potassiumsulphate, slightly in excess of 0.1 normal is provided; in this rangethe lyotropic effect is pronounced, and the potassium salts appear to bethe most advantageous. The anion associated with the potassium cation isalso important, and at concentrations in excess of 0.1 normal theeffects of the I-Iofmeister series of anions is noted, with the sulphatehaving a greater effect than most of the other conventionally employedanions. From the relation of the anions and cations in the Hofmeisterand lyotropic series, therefore, the desired contraction of theagar-agar gel may be obtained, within the limits offered by the weakestand strongest acting electrolyte.

Should the dental technician desire to prepare a series of models ofdiiierent size for a given technique, for example, he may immerse theimpression in a two per cent solution of potassium sulphate, take animpression therein, then immerse the impression in a 0.1 normal solutionof potassium nitrate for sufficient time to secure a thorough diffusionof the potassium sulphate out of the gel and a replacement thereof bythe potassium nitrate, which will cause a slight expansion of theimpression. The new model made from the thus expanded impression will beof somewhat greater size than the model made with the potassiumsulphate-treated impression. The electrolytes which may be employed arenecessarily those which are compatible with agar-agar, however, as willbe apparent to those familiar with agar-agar gels, i. e., those whichwill not cause disintegration of agar-agar gels, such as experienced forexample by the presence of a strong mineral acid. In this connectionpotassium sulphate has been found to be the least detrimental to theagar-agar gel, and this feature, taken with the beneficial behavior ofpotassium sulphate with respect to gypsum plaster as heretofore has beennoted by investigators in the art, makes the use of this electrolytehighly advantageous.

It will further be appreciated that the electrolyte need not beincorporated in the gel composition prior to the occasion of taking theimpression, inasmuch as the impression may be immersed in theelectrolyte solution which will cause the desired contraction to takeplace as the electrolyte diffuses into the gel structure. The diffusiontime in a 15% agar-agar gel has been determined to be on the order ofone-eighth inch per hour, and the impression should be immersed in theelectrolyte solution for suflicient time to secure a uniform penetrationthereof. For example, an impression having a maximum thickness ofthree-eighths inch should be immersed for a period of not less than oneand one-half hours to secure the diffusion of the electrolyte to theinner portions of the impression.

An additional feature of the invention is to be noted in connection withthe re-swelling of agar-agar hydrocolloid impressions which have beensubjected to shrinkage by moisture loss, in that such a shrunkenimpression may be immersed in the desired electrolyte solution for aperiod of 24 hours and it will imbibe the electro lyte solution andattain substantially the same dimensions as it would attain if it hadbeen immersed in the solution immediately after formation. Similarly, anexpanded impression (which has been soaked in plain water, for example)may be returned to its correct dimen-' sions by immersion in a body ofelectrolyte solution, in the same manner.

I claim:

1. The method of controlling the physical dimensions of a molded body ofdental impression hydrocolloid in gel condition and having an agar-agarbase, which comprises maintaining such a molded body immersed in anaqueous solution of an electrolyte which is compatible with agar-agargels and which will restrict the swell ing of such body to a value lessthan that to which it will swell in plain water.

2. The method of controlling the physical dimensions of a molded body ofdental impression hydrocolloid in gel condition and having an agar-agarbase, which comprises maintaining such a body immersed in an aqueoussolution containing in the neighborhood of 0.1 normal concentration ofan electrolyte which is compatible with agar-agar gels.

3. The method set forth in claim 2, said aqueone solution comprising asolution of potassium sulphate.

4. The method of controlling the physical dimensions of a molded body ofelectrolyte-containing dental impression hydrocolloid in gel conditionand having an agar-agar base, which comprises maintaining such a moldedbody in an aqueous solution of such electrolyte at a concentrationequivalent to the concentration thereof in said hydrocolloid, saidelectrolyte being one which is compatible with agar-agar gels and whichwill restrict the swelling of such a gel to a value less than that towhich it will swell in plain water.

5. The method of controlling the physical dimensions of a molded gelbody of dental impression hydrocolloid containing an agar-agar base andan electrolyte at a concentration in the neighborhood of 0.1 normal,said electrolyte being one which is compatible with. agar-agar gels,which comprises maintaining such a molded body in an aqueous solution ofsuch an electrolyte at a concentration in the neighborhood of 0.1normal.

6. The method set forth in claim 5, said electrolyte comprisingpotassium sulphate.

WALTER J. VAN ROSSEM

